Recently, to cope with environmental problems or restrain fossil energy for preventing an-increase in production of petroleum, power sources for automobiles have been reviewed. For example, an electric vehicle (hereinafter referred to as an EV) which does not use petroleum or emit CO2 and a plug-in hybrid electric vehicle (hereinafter referred to as a PHEV) in which an electric motor is combined with an internal combustion engine and the emission of CO2 is drastically reduced have spread.
Several hundred thousand to several million EVs or PHEVs are expected to spread in the future. In that case, the amount of energy that is needed to charge batteries, which are power sources for EVs, will create a huge demand for electrical power, which will possibly affect stable control of the supply-demand balance of a power system which has been used so far.
On the other hand, in order to solve the environmental problems, that is, in order to realize a low-oxygen society, large-scale introduction of renewable power sources such as solar power generation, wind power generation and the like is indispensable. However, power supply of the renewable power sources in which there is large temporal fluctuation also has a great impact on stable control of the supply-demand balance of the power system.
FIG. 1 is a diagram illustrating an example of a relationship between supply power and demand power (power that is demanded) in one day.
As illustrated in FIG. 1, the relationship between the supply power and the demand power from 0 O'clock to 24 O'clock changes over time.
It is difficult to change the base supply power output thereof (adjustability with respect to power fluctuation is small) but the base supply power can generate electric power inexpensively, and this type of power supply can be increased if demand for power increases due to spread of EVs and PHEVs.
The demand power is consumed by consumers such as general households and businesses and fluctuates depending on the condition such as seasons, weekdays and weekends. Since the demand is involved in human activities, it has a cyclic nature of 24 hours, in which demand is high during daytime and tends to fall during nighttime. The portion of a balance by which the demand power falls below the base supply power during nighttime, when the demand power is small, becomes nighttime surplus power. In order not to waste this nighttime surplus power, various methods have been examined. For example, such methods include a method in which pumped storage hydro is used during the nighttime to pump and store water as potential energy, a method in which nighttime power charges are lowered so that consumers can accumulate the power as heat in equipment such as a heat pump and can use the heat (hot water, for example) during daytime, and a method of time-shift of demand in which dedicated batteries are prepared and surplus is charged, and the charging power is discharged when power is not sufficient and the like.
Moreover, as illustrated in FIG. 1, during daytime, photovoltaic power generation becomes large, and the sum of the generated power of the base power source and the generated power of the solar power source exceeds the demand power. That is, the excess power also becomes a surplus similarly to the nighttime surplus power. This surplus power can also be effectively used without discarding generated power by means of well-designed time-shift of demand similar to the nighttime surplus power.
In general, without power generated by solar power source, thermal power generation whose output can be easily varied and the like are used for meeting the demand in a time zone when the demand power exceeds the power generated by the base power source.
Moreover, methods of accumulating surplus power include, other than a method of accumulation through conversion to mechanical energy, such as the above-described pumped-storage power generation, a method of accumulating power in a battery. Practical applications have been found for large sized batteries, such as NAS, for example, and the way in which these batteries are used is technically feasible, but the investment needed to realize practical application of these large sized batteries is high.
On the other hand, a problem which can occur when a large number of EVs and PHEVs have spread will be described below.
FIG. 2 is a diagram illustrating the simulation result of the charging power when charging 50 EVs that are used for the purpose of commuting is simulated for 3 days. This simulation was conducted under the condition in which each EV is plugged in immediately after a trip and charging is started.
As illustrated in FIG. 2, since a time zone when a trip is made in commuting is almost the same for the 50 cars, the time zone when charging is started with stop of movement is substantially equal for them. Thus, the charging power locally gathers in the concentrated time zone. From the standpoint of balance between supply and demand, power generation facilities that are required to satisfy large demand, which are steep and in which there is large fluctuation width are, in general, expensive. Also, where EVs for commuting gather locally as in a parking lot of a company, power distribution facilities which can be used for charging need to be prepared.
Moreover, a concept called smart grid has been recently proposed in which the balance between supply and demand balance can be stabilized by a collaborative arrangement among consumer devices, consumer power sources, and power systems (See Patent Literature 1, for example).
Furthermore, a technology to utilize EV batteries expected to be promoted for stable operation of power systems has been examined (See Non-Patent Literatures 1 and 2, for example).
In Non-Patent Literatures 1 and 2, a method in which not only a battery that mounted on an EV is charged by a power system but that is also discharged to the power system so as to suppress output fluctuation of a renewable power source has been proposed. This method is the ultimate method which can absorb supply-demand balance fluctuation in a short cycle such as several Hz. However, there is a cost problem since dedicated functions of charging/discharging need to be provided in each EV or another problem in which means for separating an EV from the power system without disrupting collaborative operation with a thermal power plan needs to be provided. Moreover, in view of the consideration that the main purpose of using a battery installed on an EV is to enable the vehicle to move (run), discharging energy from the power system, for the purpose of providing power to other objects, reduces the battery life of the charged battery, and is demerit for the owner of an EV.
Here, from the standpoint of the impact on the power system in which a charging operation is only needed to enable an EV or a PHEV to move, or from the standpoint of a battery for time shift of power demand so that it corresponds to the amount of surplus power, managing a charging schedule for a large number of EVs and PHEVs is expected to become indispensable.
On the other hand, the battery that is installed in an EV is means that enables an EV to move, which is the purpose of the EV, is different from a battery that is used for facilities that do not move, such as a heat pump or the like, which is typical for time shift of power demand when there is demand for power during the night at areas where the battery is not continuously connected to the power system and in which whether or not the battery is connected to the power system depends on the behavior of the EV owner which is not known. Thus it is difficult to apply a planning method or a scheduling method on the basis of stationary facilities.
As described above, in a system in which a large number of EVs are linked and charged, it is necessary to control charging of batteries mounted on each of the EVs in real time, taking into consideration that situations are in a state of constant change. In this regard, hardly any comprehensive consideration have been giving to proposing a practical system to reduce the load/risk for the EV owner (insufficient battery charge when the vehicle moves, accelerated battery energy loss in the EV) and to reduce calculation processing loads for optimal scheduling.